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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

In Vivo Cell Wall Loosening by Hydroxyl Radicals during Cress Seed Germination and Elongation Growth^1,[W],[OA]

University of Freiburg, Faculty of Biology, Institute for Biology II, Botany/Plant Physiology, D–79104 Freiburg, Germany (K.M., A.L., G.L.-M.); Edinburgh Cell Wall Group, Institute of Molecular Plant Sciences, University of Edinburgh, Edinburgh EH9 3JH, United Kingdom (R.A.M.V., S.C.F.); and Commissariat à l'Energie Atomique, Institut de Biologie et de Technologies de Saclay, CNRS URA 2096, Service de Bioénergétique, Biologie Structurale et Mécanisme, 91191 Gif-sur-Yvette cedex, France (A.K.-L.)

Loosening of cell walls is an important developmental process in key stages of the plant life cycle, including seed germination, elongation growth, and fruit ripening. Here, we report direct in vivo evidence for hydroxyl radical (^·OH)-mediated cell wall loosening during plant seed germination and seedling growth. We used electron paramagnetic resonance spectroscopy to show that ^·OH is generated in the cell wall during radicle elongation and weakening of the endosperm of cress (Lepidium sativum; Brassicaceae) seeds. Endosperm weakening precedes radicle emergence, as demonstrated by direct biomechanical measurements. By ³H fingerprinting, we showed that wall polysaccharides are oxidized in vivo by the developmentally regulated action of apoplastic ^·OH in radicles and endosperm caps: the production and action of ^·OH increased during endosperm weakening and radicle elongation and were inhibited by the germination-inhibiting hormone abscisic acid. Both effects were reversed by gibberellin. Distinct and tissue-specific target sites of ^·OH attack on polysaccharides were evident. In vivo ^·OH attack on cell wall polysaccharides were evident not only in germinating seeds but also in elongating maize (Zea mays; Poaceae) seedling coleoptiles. We conclude that plant cell wall loosening by ^·OH is a controlled action of this type of reactive oxygen species.

² Present address: Wageningen UR, Agrotechnology & Food Sciences Group, Postharvest Quality and Technology, P.O. Box 17, 6700 AA Wageningen, The Netherlands.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Gerhard Leubner-Metzger (gerhard.leubner{at}biologie.uni-freiburg.de).

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www.plantphysiol.org/cgi/doi/10.1104/pp.109.139204

^* Corresponding author; e-mail gerhard.leubner{at}biologie.uni-freiburg.de.

Received April 9, 2009; accepted May 29, 2009; published June 3, 2009.

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